docs: Improve pg_settings_get_flags docs.

In the docs, the GUC flags that pg_settings_get_flags() reported were
listed using <simplelist>. But the list was treated as separate lines
in the existing function table and didn't look good. For better view,
this commit separates the list from the table entry for
pg_settings_get_flags() and adds the table for it at the bottom of
the existing function table.

Author: Fujii Masao
Reviewed-by: Alvaro Herrera, Michael Paquier
Discussion: https://postgr.es/m/f093edf9-6e5a-b119-ee50-6a2c97c79ee8@oss.nttdata.com

Back-patch of f2d0c7f18 into v15.
Discussion: https://postgr.es/m/20221103123320.GQ16921@telsasoft.com

doc/src/sgml/func.sgml

@@ -23915,28 +23915,8 @@ SELECT currval(pg_get_serial_sequence('sometable', 'id'));
         Returns an array of the flags associated with the given GUC, or
         <literal>NULL</literal> if it does not exist. The result is
         an empty array if the GUC exists but there are no flags to show.
-        Only the most useful flags are exposed, as of the following:
+        Only the most useful flags listed in
-        <simplelist>
+        <xref linkend="functions-pg-settings-flags"/> are exposed.
-         <member>
-          <literal>EXPLAIN</literal>: parameters included in
-          <command>EXPLAIN (SETTINGS)</command> commands.
-         </member>
-         <member>
-          <literal>NO_SHOW_ALL</literal>: parameters excluded from
-          <command>SHOW ALL</command> commands.
-         </member>
-         <member>
-          <literal>NO_RESET_ALL</literal>: parameters excluded from
-          <command>RESET ALL</command> commands.
-         </member>
-         <member>
-          <literal>NOT_IN_SAMPLE</literal>: parameters not included in
-          <filename>postgresql.conf</filename> by default.
-         </member>
-         <member>
-          <literal>RUNTIME_COMPUTED</literal>: runtime-computed parameters.
-         </member>
-        </simplelist>
        </para></entry>
       </row>
 
@@ -24344,6 +24324,46 @@ SELECT collation for ('foo' COLLATE "de_DE");
    </tgroup>
   </table>
 
+  <table id="functions-pg-settings-flags">
+   <title>GUC Flags</title>
+   <tgroup cols="2">
+    <thead>
+     <row><entry>Flag</entry><entry>Description</entry></row>
+    </thead>
+    <tbody>
+     <row>
+      <entry><literal>EXPLAIN</literal></entry>
+      <entry>Parameters with this flag are included in
+       <command>EXPLAIN (SETTINGS)</command> commands.
+      </entry>
+     </row>
+     <row>
+      <entry><literal>NO_SHOW_ALL</literal></entry>
+      <entry>Parameters with this flag are excluded from
+       <command>SHOW ALL</command> commands.
+      </entry>
+     </row>
+     <row>
+      <entry><literal>NO_RESET_ALL</literal></entry>
+      <entry>Parameters with this flag are excluded from
+       <command>RESET ALL</command> commands.
+      </entry>
+     </row>
+     <row>
+      <entry><literal>NOT_IN_SAMPLE</literal></entry>
+      <entry>Parameters with this flag are not included in
+       <filename>postgresql.conf</filename> by default.
+      </entry>
+     </row>
+     <row>
+      <entry><literal>RUNTIME_COMPUTED</literal></entry>
+      <entry>Parameters with this flag are runtime-computed ones.
+      </entry>
+     </row>
+    </tbody>
+   </tgroup>
+  </table>
+
   <para>
    <xref linkend="functions-info-object-table"/> lists functions related to
    database object identification and addressing.